Use one query to check email uniqueness

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/503

Reduce the number of queries when checking if email is unique. This change aims to help improve the response time of sign-up requests.

Before

!emails.exists?(email: email) && Email.exists?(email: email)

-- code: !emails.exists?(email: email)
SELECT
    1 AS one
FROM
    "emails"
WHERE
    "emails"."user_id" = 332
    AND "emails"."email" = 'test+3@ex.com'
LIMIT 1

-- code: Email.exists?(email: email)
SELECT
    1 AS one
FROM
    "emails"
WHERE
    "emails"."email" = 'test+3@ex.com'
LIMIT 1

After

Email.where.not(user: self).where(email: email).exists?

SELECT
    1 AS one
FROM
    "emails"
WHERE
    "emails"."user_id" != 332
    AND "emails"."email" = 'test+3@ex.com'
LIMIT 1

EXPLAIN

https://console.postgres.ai/shared/69d3df92-ee0d-454e-912d-ae8d8915bacb

 Limit  (cost=0.56..3.58 rows=1 width=4) (actual time=12.005..12.007 rows=0 loops=1)
   Buffers: shared read=4
   I/O Timings: read=11.935 write=0.000
   ->  Index Scan using index_emails_on_email on public.emails  (cost=0.56..3.58 rows=1 width=4) (actual time=12.002..12.003 rows=0 loops=1)
         Index Cond: ((emails.email)::text = 'eugie_new_email@gmail.com'::text)
         Filter: (emails.user_id <> 9670031)
         Rows Removed by Filter: 0
         Buffers: shared read=4
         I/O Timings: read=11.935 write=0.000

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Before	After

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